Anti-scattering x-ray shielding for ct scanners

ABSTRACT

A CT scanner comprising a stator and a rotor having an axis of rotation mounted to the stator so that the rotor is rotatable about the axis of rotation comprising: an X-ray source mounted to the rotor, said X-ray source having a focal spot from which X-rays emanate; an X-ray detector array comprising a matrix of rows and columns of X-ray detectors; anti-scattering (AS) material for absorbing X-rays positioned between columns of the X-ray detectors; and anti-scattering (AS) material for absorbing X-rays positioned between rows of the X-ray detectors, whereby the AS material is located between every other row and/or column of detectors, respectively. Furthermore, the thickness and/or height of the foils between rows may be different from the thickness and/or height of the foils between columns.

FIELD OF THE INVENTION

The present invention relates to computerized tomography (CT) X-rayimaging and in particular to methods for shielding X-ray detectors in aCT imaging system from scattered X-rays.

BACKGROUND OF THE INVENTION

In CT X-ray imaging of a patient, X-rays are used to image internalstructure and features of a region of the person's body. The imaging isperformed by a CT-imaging system, hereinafter referred to as a “CTscanner” that images internal structure and features of a plurality ofcontiguous, relatively thin planar slices of the body region usingX-rays.

The CT scanner generally comprises an X-ray source that provides aplanar, fan-shaped X-ray beam emanating from a focal spot of the X-raysource and an array of closely spaced X-ray detectors that aresubstantially coplanar with the fan-beam and face the X-ray source. TheX-ray source and array of detectors are mounted in a gantry so that aperson being imaged with the CT scanner, generally lying on anappropriate support couch, can be positioned within the gantry betweenthe X-ray source and the array of detectors. The gantry and couch aremoveable relative to each other so that the X-ray source and detectorarray can be positioned axially, along a “z-axis”, at desired locationsalong the patient's body.

The gantry comprises a stationary structure, referred to as a stator,and a rotary element, referred to as a rotor, which is mounted to thestator so that the rotor is rotatable about the z-axis. In thirdgeneration CT scanners the X-ray source and detectors are mounted to therotor. The detectors are generally arrayed along an arc of a circlehaving its plane perpendicular to the z-axis and its center located at afocal spot of the scanner's X-ray source. Hereinafter, such an array ofdetectors along an arc of a circle is referred to as a “row” ofdetectors. Angular position of the rotor about the z-axis iscontrollable so that the X-ray source can be positioned at desiredangles, referred to as “view angles”, around the patient's body. Infourth generation CT scanners the X-ray detector array comprisesdetectors positioned around the perimeter of a circle to form a fullcircle of detectors. The circle of detectors is stationary and the X-raysource is mounted to the rotor and rotates with the rotor.

To image a slice in a region of a patient's body, the X-ray source ispositioned at the z-axis coordinate of the slice and rotated around theslice through an angle of at least 180° to illuminate the slice withX-rays from a plurality of different view angles. At each view angle,detectors in the array of detectors measure intensity of X-rays from thesource that pass through the slice. The intensity of X-rays measured bya particular detector in the array of detectors is a function of anamount by which X-rays are attenuated by material in the slice along astraight-line path length from the X-ray source, through the particularslice, to the detector.

The measurements provided by the X-ray detectors are processed usingalgorithms known in the art to provide a map of the absorptioncoefficient of the material in the slice as a function of position. Mapsof the absorption coefficient for the plurality of contiguous slices inthe region of the patient's body are used to display and identifyinternal organs and features of the region.

Many older CT scanners are single slice scanners that comprise only asingle row (i.e. also a single circle for fourth generation CT scanners)of X-ray detectors and image only one slice of a region of a person'sbody at a time. Most modem CT scanners are multislice CT scannersdesigned to simultaneously image a plurality of slices of a patient. Amultislice CT scanner comprises a plurality of parallel rows (or circlesfor fourth generation scanners) of X-ray detectors closely spaced onenext to the other along the z-axis. The detector array of a multisliceCT scanner is thus a “two-dimensional” (ignoring the curvature of therows) matrix of rows and columns of detectors. A column of detectors inthe array refers to detectors comprised in the rows of detectors thatlie along a same line parallel to the z-axis of the scanner.

A multislice scanner can be operated to simultaneously image a number ofslices of a patient up to a maximum number of slices equal to the numberof rows of detectors. Typically, signals from detectors in a multislicescanner are combined in accordance with any of various algorithms knownin the art to simultaneously image a plurality of slices that is lessthan the number of rows of detectors. A present day conventionalmultislice scanner may image as many as four to sixteen slices of aregion of a patient simultaneously and scanners that simultaneouslyimage more than sixteen slices are under development.

Ideally, each detector in a CT scanner measures intensity of X-rays thatreach the detector after passage along a substantially straight-linepath from the X-ray source to the detector. Therefore, ideally, onlythose X-rays that are neither absorbed by the material along the pathfrom the X-ray source to the detector nor scattered by the material atangles that prevent the X-rays from being incident on the detector reachthe detector. However, X-rays that are scattered out of the path fromthe X-ray source to one X-ray detector in the detector array of the CTscanner are generally scattered in directions towards other X-raydetectors in the scanner detector array. If these scattered X-rays areincident on the other X-ray detectors, they can generate error inmeasurements provided by the other detectors and degrade quality of animage provided by the CT scanner.

To reduce “scattering errors” in a CT scanner, X-ray detectors in thescanner's detector array are generally shielded from scattered X-rays.“Anti-scattering” shielding, hereinafter “AS shielding”, for a detectorarray of a multislice CT scanner usually comprises a thin planar bafflefoil, hereinafter an “AS foil”, formed from a suitable X-ray absorbingmaterial positioned between each column of detectors in the array. Theplane of each AS foil is parallel to the z-axis and oriented so that itintersects the focal spot of the X-ray source, or a centroid of thefocal spot if a deflecting focal spot is used. Hereinafter, a focal spotof an X-ray source is understood to mean a focal spot or, for a CTscanner having a deflecting focal spot, the centroid of the focal spot.For single slice CT scanners the AS foils are relatively effective inmoderating effects of scattered X-rays on detector accuracy and imagequality. However, the X-ray fan-beam in a multislice scanner issubstantially thicker along the z-axis than the X-ray fan-beam in singleslice scanners. As a result, detectors in a multislice CT scanner arepotentially exposed to substantially more sources of scattered X-raysand thereby to a greater flux of scattered X-rays than are detectors ina single slice CT scanner.

To cope with increased flux of scattered X-rays in a multislice CTscanner, AS foils in a multislice scanner are often made higher than ASfoils in a typical single slice scanner. However, a height to which ASfoils can be fabricated is limited by production tolerances. As theheight of AS foils is increased, the foils have to be fabricated andpositioned to ever-finer tolerances. For example, as the height of ASfoils in a CT scanner increases, the AS foils have to be aligned withthe focal spot of the scanner's X-ray source to a greater degree ofaccuracy. To an extent that an AS foil is misaligned with the focal spotor its surface is compromised by inhomogeneities, it may shadow to agreater extent X-ray detectors adjacent to the AS foil. Shadowing altersthe detection efficiency of a shadowed X-ray detector and may generateartifacts in images generated by the multislice CT scanner. In addition,shadowing, when it exists, is often unstable and may change from time totime. As a result, it can be difficult to compensate images provided bythe CT scanner for artifacts introduced by shadowing.

It is noted that because of the geometry of fourth generation CTscanners, it is generally not possible to provide anti-scatteringshielding for detectors comprised in a fourth generation CT scanner thatdoes not result in undesirable shadowing of the detectors. As a resultanti-scattering shielding is usually not provided for fourth generationscanners.

For multislice scanners, as the number of slices simultaneously imagedincreases, the problem of scattering error is exacerbated As it does notseem possible to deal with the problem simply by increasing the heightof AS foils used to shield the detectors, new solutions to scatteringerror are required

SUMMARY OF THE INVENTION

An aspect of some embodiments of the present invention relates toproviding an improved system for shielding X-ray detectors in a detectorarray of a multislice CT scanner from scattered X-rays.

An aspect of some embodiments of the present invention relates toproviding AS shielding for X-ray detectors in a detector array of amultislice CT scanner comprising AS foils along at least two differentdirections.

A direction of an AS foil of a CT scanner is defined as a direction of aline of intersection of the plane of the AS foil with the surface of theX-ray detector array. For convenience, features and elements on thesurface of the detector array are located using two coordinates, az-coordinate measured along the z-axis and an “s” coordinate measuredalong a line, an “s-axis”, in the surface of the detector array parallelto the rows of detectors in the array. Direction of an AS foil, i.e. thedirection of its line of intersection with the X-ray detector array, isdetermined with reference to the z and s axes. Prior art AS foilsdescribed above, which are parallel to the columns of detectors in a CTscanner detector array, are therefore “z-axis” AS foils.

In an embodiment of the present invention, AS shielding comprises s-axisAS foils parallel to the rows of the detectors in the multislicescanner's detector array as well as z-axis column AS foils. For a givenmaximum AS foil height, the added s-axis AS foils provide efficientadded shielding against scattered X-rays resulting from the relativelylarge dimension, thickness, of the fan-beam of the multislice scanneralong the z-axis.

According to an aspect of some embodiments of the present invention, foreach X-ray detector, from a perspective of an origin of a coordinatesystem located at the center of the X-ray detector, the AS shieldingconfiguration is similar.

Configuration similarity of the AS shielding reduces possible artifactsin an image provided by the multislice scanner that might result fromdifferences in the energy spectrum of X-rays reaching differentdetectors caused by differences in the AS shielding protecting thedifferent detectors. In accordance with some embodiments of the presentinvention, to within a parity transformation and/or a rotationtransformation, the AS shielding of any two X-ray detectors of the samesize and shape, as seen from their respective coordinate systems, aresubstantially identical.

There is therefore provided, in accordance with an embodiment of thepresent invention, a CT scanner comprising a stator and a rotor havingan axis of rotation mounted to the stator so that the rotor is rotatableabout the axis of rotation comprising: an X-ray source mounted to therotor, said X-ray source having a focal spot from which X-rays emanate;an X-ray detector array comprising a matrix of rows and columns of X-raydetectors; anti-scattering (AS) material for absorbing X-rays positionedbetween columns of the X-ray detectors; and anti-scattering (AS)material for absorbing X-rays positioned between rows of the X-raydetectors.

Optionally, as seen from a perspective of a first coordinate systemlocated in substantially any first detector of the detector array and ahomologous coordinate system located in substantially any seconddetector of the detector array, the AS material has substantially a sameconfiguration to within a parity transformation and/or a rotationtransformation

Optionally, the AS material is located between every other row ofdetectors. Additionally or alternatively, the AS material is locatedbetween every other column of detectors.

In some embodiments of the present invention the AS material is formedin a shape of a thin foil for which for any point on the foil a segmentof a line from the focal point of the X-ray source to the point liessubstantially within or on the surface of the foil.

Optionally, foil between columns of detectors extends towards the focalpoint to a height relative to the detector array that is different froma height to which foil located between columns of detectors extendstowards the focal point. Additionally or alternatively, thickness of thefoil between rows is different from thickness of the foil betweencolumn.

In some embodiments of the present invention, the detectors have ahexagonal shape and the foil is shaped to follow the hexagonal shape ofthe detectors.

BRIEF DESCRIPTION OF FIGURES

Non-limiting examples of embodiments of the present invention aredescribed below with reference to figures attached hereto and listedbelow. In the figures, identical structures, elements or parts thatappear in more than one figure are generally labeled with a same numeralin all the figures in which they appear. Dimensions of components andfeatures shown in the figures are chosen for convenience and clarity ofpresentation and are not necessarily shown to scale.

FIG. 1A schematically shows a third generation multislice CT scanner andits detector array, in accordance with prior art, but without AS foilsthat are used to shield X-ray detectors in the array;

FIG. 1B schematically shows the X-ray detector array of the CT scannershown in FIG. 1A and AS shielding used to shield X-ray detectors in thearray, in accordance with prior art;

FIG. 1C schematically shows a plan view of the detector array and ASshielding shown in FIG. 1B, in accordance with prior art;

FIG. 2 schematically shows an X-ray detector array of a CT scannerhaving AS shielding, in accordance with an embodiment of the presentinvention;

FIG. 3A schematically shows a plan view of the detector array and ASshielding shown in FIG. 2 and a configuration of z-axis and s-axis ASfoils in the AS shielding, in accordance with an embodiment of thepresent invention;

FIG. 3B schematically shows a plan view of an X-ray detector array andAS shielding having an alternative configuration of z-axis and s-axis,in accordance with an embodiment of the present invention;

FIG. 3C schematically shows a plan view of an X-ray detector array andAS shielding having another alternative configuration of z-axis ands-axis, in accordance with an embodiment of the present invention;

FIG. 3D schematically shows a partially cutaway perspective view of anX-ray detector array and AS shielding having a configuration of z-axisand s-axis AS foils in which z-axis and s-axis foils have differentheights, in accordance with an embodiment of the present invention; and

FIG. 3E schematically shows a partially cutaway perspective view of anX-ray detector array having hexagonal X-ray detectors and AS shieldingtherefore, in accordance with an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1A schematically shows a third generation multislice CT scanner 20,in accordance with prior art. Only those features and components of CTscanner 20 germane to the present discussion are shown in FIG. 1A.

CT scanner 20 comprises an X-ray source 22 controllable to provide anX-ray fan-beam 24, schematically indicated by dashed lines 26, and anarray 28 of X-ray detectors 30. Fan-beam 24 emanates from a focal region32, hereinafter referred to as “focal spot 32”, of X-ray source 22.X-ray source 22 and detector array 28 are mounted to a rotor 40, whichin turn is rotatably mounted to a stator 42 so that the rotor can berotated about an axis 44 conveniently labeled as the z-axis of acoordinate system 45. Stator 42 and rotor 40 are components of a gantry46 of CT scanner 20.

Array 28 has columns 50 and rows 52 of X-ray detectors 30. Array 28 isshown having four rows 52 of detectors to indicate, by way of example,that CT scanner 20 is a multislice scanner capable of imaging up to fourslices of a patient at a time. Each row 52 of detectors is a curved rowthat lies substantially along an arc of a circle having its planeperpendicular to the z-axis and its center at focal spot 32 of X-raysource 22. A feature of X-ray detector array 28 is conveniently locatedby a z-coordinate determined with reference to the z-axis and an“s-coordinate” determined with reference to an s-axis shown alongsidearray 28. The s-axis is an arc of a circle having a same radius andcenter as the arc of the circle along which detectors in a row 52 ofdetectors are positioned. AS foils that are used to shield X-raydetectors 30 and reduce effects of scattered X-rays are not shown inFIG. 1A so as to display in FIG. 1A the configuration of X-ray detectors30 in array 28. The AS foils are shown in FIG. 1B, discussed below.

It is noted that the number and relative size of X-ray detectors 30shown in detector array 28 is arbitrary and chosen for convenience andclarity of presentation. In practice, an X-ray detector array in atypical multi slice CT scanner may comprise many thousands of smallX-ray detectors having areas typically equal to about a squaremillimeter.

A patient to be imaged by CT scanner 20 is supported on a couch 48.Couch 48 is mounted on a suitable pedestal (not shown) and iscontrollable to be translated axially along z-axis 44 so as to positiona region of the patient's body to be imaged by CT scanner 20 insidegantry 46, between X-ray source 22 and array 28. When the region to beimaged is properly positioned inside gantry 46, rotor 40 is controlledto rotate X-ray source 22 around the z-axis to illuminate the regionwith X-rays from a plurality of view angles. For each view angle, X-raydetectors 30 in array 28 generate signals responsive to intensity ofX-rays from X-ray source 22 that pass through the region and that areincident on the detectors. The signals generated by X-ray detectors 30in a same row 52 of detectors 30 are processed to generate an image of aslice of the region.

Ideally, each detector 30 measures intensity only of X-rays from X-raysource 22 that travel along a substantially straight-line path from theX-ray source through the region being imaged to the detector. To reduceerrors in intensity measurements provided by detectors 30, which aregenerated by X-rays that do not reach the detectors along straight-linepaths but reach the detectors after being scattered, AS foils are usedto shield X-ray detectors 30 in array 28.

FIG. 1B schematically shows X-ray detector array 28 of CT scanner 20shown in FIG. 1A and AS shielding 60 used to shield detectors 30 in thearray in accordance with prior art. Also shown are a portion of rotor 40of CT scanner 20 to which detector array 28 is mounted, and otherfeatures of CT scanner 20 germane to the discussion.

AS shielding 60 comprises an AS foil 62 placed between each column 50(FIG. 1A) of detectors 30. Each AS foil 62, hereinafter referred to as a“z-foil”, has its plane parallel to the z-axis and is oriented so thatthe plane intersects focal spot 32. To schematically illustrate theorientation of z-foils 62 relative to focal spot 32, for each of a fewz-foils 62 a dashed line 64 coplanar with the z-foil extends from thez-foil to intersect focal spot 32. Typically, z-foils 62 are formed fromTungsten or Molybdenum foil having thickness in a range from about 0.05millimeters to about 0.2 millimeters and extend from array 28 towardsfocal spot 32 to a height in a range from about 20 millimeters to about40 millimeters.

As z-foils 62 are made higher, planarity, thickness and orientation ofthe foils must be controlled to finer tolerances in order to preventundesirable shadowing of X-ray detectors 30 by the foils. Practically,for X-ray detectors having characteristic dimensions of about amillimeter, a maximum height to which z-foils 62 can be made is about 40millimeters. As X-ray detectors become smaller, planarity, thickness andorientation of AS foils also have to be controlled to more stringenttolerances.

FIG. 1C shows a schematic plan view of a region 70 of detector array 28shown in FIGS. 1A and 1B and z-foils 62, shown in FIG. 1B, associatedwith the region. For simplicity of presentation, region 70 of detectorarray 28 is shown in FIG. 1C as being planar. X-ray detectors 30 indetector array 28 that are comprised in region 70 are represented bylightly shaded squares and the z-axis and s-axis shown in FIGS. 1A and1B are shown in FIG. 1C to orient region 70. Columns 50 of detectors 30in region 70 are parallel to the z-axis and portions of rows 52 ofdetectors 30 that are comprised in the region are parallel to thes-axis.

A relatively narrow “isolation gap” 72 separates adjacent detectors 30in a same column 50 of detectors 30. The width of isolation gaps 72 isusually made as small as possible in order to provide a packing densityof X-ray detectors as large as possible. Typically, the width ofisolation gaps 72 ranges from about 0.05 to about 0.3 millimeters.

Z-foils 62 are made as thin as possible consistent with effectiveshielding of X-ray detectors 30 to minimize space between columns 50 ofthe detectors. Motivation for minimizing thickness of z-foils 62 is tominimize the footprint of z-foils 62 on detector array 28 and providefor as large as possible a packing density of X-ray detectors 30 inarray 28. As noted above, z-foils 62 are usually fabricated fromTungsten or Molybdenum and typically have a thickness in a range fromabout 0.05 to about 0.2 mm. As a result, adjacent columns 50 ofdetectors 30 are separated by a gap having a width generally in a rangefrom about 0.2 to about 0.3 mm.

FIG. 2 schematically shows an X-ray detector array 80 of a CT scanner(only parts of which are shown) and AS shielding 82 used to shielddetectors 30 in the array, in accordance with an embodiment of thepresent invention. A portion of a rotor 40 of the CT scanner to whichdetector array 80 is mounted and the scanner's X-ray source 22 are alsoshown.

X-ray detector array 80 is similar to X-ray detector array 28 shown inFIG. 1A and FIG. 1B and detector array 80 is a two-dimensional array ofcolumns 50 of X-ray detectors 30 parallel to the z-axis and rows 52 ofX-ray detectors 30 parallel to the s-axis. Rows 52 and columns 50 ofX-ray detectors 30 are not shown in the perspective of FIG. 2, but areshown in FIGS. 3A-3D discussed below, which show plan views of a regionof detector array 28.

Whereas AS shielding 60 used with detector array 28 has AS foils, i.e.z-foils 62, (FIG. 1B and 1C) along a single direction, AS shielding 82,in accordance with an embodiment of the present invention, comprises ASfoils aligned along at least two different directions. (As noted above,the direction of an AS foil is defined by the direction of theintersection line of the foil's plane with the surface of the detectorarray, as determined with reference to the z and s axes.) AS shielding82 comprises AS foils 84, hereinafter referred to as “s-foils 84”,parallel to the s-axis as well as z-foils 86, which are parallel to thez-axis. In accordance with an embodiment of the present invention, z ands-foils 86 and 84 are configured so that there is a z-foil 86 locatedbetween every other column 50 (FIGS. 3A-3D) of X-ray detectors 30 and ans-foil located between every other row 52 (FIGS. 3A-3D) of the X-raydetectors.

It is noted that whereas constructing an AS foil configuration havingfoils along two directions is generally more difficult than constructingan AS foil configuration in which the foils are along a same singledirection, a “two-dimensional” foil configuration can be constructedusing methods known in the art For example, a two-dimensional foilconfiguration comprising foils along two directions can be constructedby appropriately slotting the foils so that they can be inserted oneinto the slots of the other to form an array of “cubicles”.

FIG. 3A schematically shows a plan view of a region 88 of detector array80. Columns 50 of detectors 30 are parallel to the z-axis shown at theright of region 88 and rows 52 of detectors 30 are parallel to thes-axis shown at the bottom of region 88. Whereas by way of example,detector array 80 is assumed to have at least eight rows 52 of detectors30, it is noted that the present invention can be practiced withdetector arrays having other than eight rows of detectors.

Shaded bands 84 represent s-foils, which are located between every otherrow 52 of detectors 30 and shaded bands 86 represent z-foils, which arelocated between every other column 50 of detectors 30. Isolation gaps 72separate adjacent detectors 30 that are not separated by a z-foil 86 oran s-foil 84.

Optionally, z-foils 86 and s-foils 84 have a same thickness and arerepresented by shaded bands having a same width Optionally, z-foils 86and s-foils 84 have a same height Z-foils 86 and s-foils 84 partitionX-ray detectors 30 in detector array 80 into groups of four detectors.Each detector 30 in a group of detectors 30 has one edge adjacent to az-foil 86 and one edge adjacent to an s-foil 84. Each of the other twoedges of each detector 30 is adjacent to an isolation gap 72.

As a result of the configuration of z-foils 86 and s-foils 84, eachX-ray detector 30 in array 28 is shielded by a substantially sameconfiguration of AS foils. To within a rotation transformation, theconfigurations of AS foils 86 and 84 seen by any two detectors 30 indetector array 80 are substantially the same. Therefore, changes that ASshielding 82 might introduce into the intensity of direct or scatteredX-rays reaching X-ray detectors 30, which could generate differenteffective detection efficiencies or “responsivities” for different X-raydetectors, are moderated.

AS shielding for a CT scanner detector array, in accordance withembodiments of the present invention, is optionally configured toprovide an appropriate degree of symmetry so that the shielding does notgenerate substantial inhomogeneities in the spectra of X-rays reachingdetectors in the array. Optionally, in AS shielding provided for a CTscanner, in accordance with an embodiment of the present invention, theconfiguration of AS foils comprised in the shielding that is seen by anytwo detectors in the CT scanner are substantially the same to within arotation and/or parity transform.

Whereas, the configuration of z-foils 86 and s-foils 84 shown in FIG. 3Aexhibits the optional degree of symmetry discussed in the precedingparagraph, the configuration shown in FIG. 3A is not the onlyconfiguration of AS foils, in accordance with embodiments of the presentinvention, that exhibits a desired degree of symmetry. AS shielding inaccordance with embodiments of the present invention, havingconfigurations of z-foils and s-foils different from that shown in FIG.3A can be used to provide each detector 30 in detector array 80 with asubstantially same configuration of AS foils.

For example, FIG. 3B schematically shows a plan view of a region 130 ofa detector array similar to detector array 80 but having a configurationof z-foils 132 and s-foils 134, for which there is an s-foil betweenevery row 52 of detectors 30 and between every other column 50 of thedetectors. Detector arrays, in accordance with an embodiment of thepresent invention, for which there is an s-foil between every other rowof detectors and a z-foil between every other column of detectors, is ofcourse possible as well.

By way of another example, FIG. 3C shows a plan view of a region 90 of adetector array similar to detector array 80 but having a configurationof z-foils 92 and s-foils 94, for which the thickness of s-foils 94 isdifferent from the thickness of z-foils 92. By way of example, in FIG.3C s-foils 94 are thinner than z-foils 92. For the AS foil configurationshown in FIG. 3C, to within a parity and rotation transformation theconfiguration of AS foils seen by any two detectors 30 in region 90 isidentical.

To illustrate, detectors 30 in a group of four detectors along the lowerright corner of region 90 are labeled D1-D4. Detector D1 is providedwith a “local” coordinate system having x′ and y′ axis as shown and a z′axis (not shown) perpendicular to the plane of the figure and pointingtowards the reader. Detector D1 has an AS shielding configuration inwhich a portion of a thin s-foil 94 is adjacent the detector's bottomedge and a portion of a thick z-foil 92 is adjacent the detector's leftedge. D2 on the other hand has an AS shielding configuration for which aportion of a thin s-foil 94 is adjacent the detector's top edge and aportion of a thick z-foil 92 is adjacent the detector's left edge. Ifthe portions of z-foil 92 and s-foil 94 adjacent the left and bottomedges of detector D1 are rotated by 180° about the z′-axis in thedetector's local coordinate system, the z-foil and s-foil portions willend up respectively adjacent the top and right edges of detector D1. Byswitching the s-foil portion, which is after the 180° rotation locatedalong the right edge of D1, to the left edge of D1, (i.e. a paritytransform), D1 will have the same configuration of AS foils as detectorD2. The shielding configuration of any detector D1-D4 can be transformedby a suitable rotation and/or parity transformation to the shieldingconfiguration of any of the other detectors D1-D4.

FIG. 3D shows a perspective partially cutaway view of AS shielding 100for a region of a detector array 102 similar to detector array 80, inwhich z-axis and s-axis foils have different heights, in accordance withan embodiment of the present invention.

AS shielding 100 comprises z-foils 104 placed between every other column50 of detectors 30 and s-foils 106 placed between every other row 52 ofdetectors 30. By way of example, the height of s-foils 106 is less thanthe height of z-foils 104. As in the case of the configuration of ASfoils (z-foils 92 and s-foils 94) shown in FIG. 3C, for AS shielding 100a configuration of z-foils 104 and s-foils 106 seen by any two detectors30 is substantially the same to within a rotation and/or paritytransform.

It is noted that in the examples of detector arrays and AS shieldingshown in FIGS. 1A-3D, X-ray detectors in the detector arrays are squareand have the same size. X-ray detectors in a CT scanner detector arraymay have shapes other than square, for example the detectors are oftenrectangular. Furthermore, a CT scanner detector array may comprise X-raydetectors having different shapes and or sizes. For example a detectorarray may comprise both square and rectangular X-ray detectors. Practiceof the present invention is not limited to detector arrays comprisingsquare detectors or detector arrays having detectors all of the samesize and/or shape. The present invention may be practiced with CTscanner detector arrays comprising other than square detectors anddetector arrays comprising detectors having different sizes and/orshapes. Symmetry considerations for configurations of AS shielding inaccordance with embodiments of the present invention discussed above fordetector arrays comprising square X-ray detectors apply equally well todetector arrays comprising non-square X-ray detectors and detectorarrays comprising detectors of different size and/or shape.

FIG. 3E schematically shows a configuration of AS shielding, 120 inaccordance with an embodiment of the present invention, for an array 122of detectors 124 in which detectors 124 are hexagonal. AS shielding, inaccordance with an embodiment of the present invention, as seen by anydetector 122, to within a rotation or parity transform is substantiallythe same for each detector.

It is also noted that whereas FIGS. 1A, 1B and 2 schematically showthird generation CT scanners and detector arrays and/or parts thereofand FIGS. 3A-3D refer to detector arrays comprised in or similar to thedetector arrays shown in FIGS. 1A, 1B and 2, the present invention isapplicable to fourth generation as well as third generation CT scannersand X-ray detector arrays. FIGS. 3A-3D and discussions thereof, as notedabove, are interpreted as referring to fourth generation detector arrayswith rows or portions of rows of detectors referring to circles ofdetectors or portions of circles of detectors.

In the description and claims of the present application, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of members, components, elements or parts of thesubject or subjects of the verb.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to limit the scope of the invention. The described embodimentscomprise different features, not all of which are required in allembodiments of the invention. Some embodiments of the present inventionutilize only some of the features or possible combinations of thefeatures. Variations of embodiments of the present invention that aredescribed and embodiments of the present invention comprising differentcombinations of feature noted in the described embodiments will occur topersons of the art. The scope of the invention is limited only by thefollowing claims.

1. A CT scanner comprising a stator and a rotor having an axis ofrotation mounted to the stator so that the rotor is rotatable about theaxis of rotation comprising: an X-ray source mounted to the rotor, saidX-ray source having a focal spot from which X-rays emanate; an X-raydetector array comprising a matrix of rows and columns of X-raydetectors; anti-scattering (AS) material for absorbing X-rays positionedbetween columns of the X-ray detectors; and anti-scattering (AS)material for absorbing X-rays positioned between rows of the X-raydetectors.
 2. A CT scanner according to claim 1, wherein as seen from aperspective of a first coordinate system located in substantially anyfirst detector of the detector array and a homologous coordinate systemlocated in substantially any second detector of the detector array, theAS material has substantially a same configuration to within a paritytransformation and/or a rotation transformation.
 3. A CT scanneraccording to claim 2 wherein the AS material is located between everyother row of detectors.
 4. A CT scanner according to claim 2 wherein theAS material is located between every other column of detectors.
 5. A CTscanner according to claim 2 wherein the AS material is formed in ashape of a thin foil for which for any point on the foil a segment of aline from the focal point of the X-ray source to the point liessubstantially within or on the surface of the foil.
 6. A CT scanneraccording to claim 5 wherein foil between columns of detectors extendstowards the focal point to a height relative to the detector array thatis different from a height to which foil located between columns ofdetectors extends towards the focal point.
 7. A CT scanner according toclaim 5 wherein thickness of the foil between rows is different fromthickness of the foil between column.
 8. A CT scanner according to claim5 wherein the detectors have a hexagonal shape and the foil is shaped tofollow the hexagonal shape of the detectors.